1. Field of the Invention
This invention relates to fasteners, especially blind fasteners, such as rivets which are frequently used in the aerospace industry and in particular in fastening or joining aerodynamic and non-aerodynamic surfaces to structural members of an aircraft.
2. Description of the Prior Art
Blind fasteners are used in a variety of applications wherein access to the blind-side of surfaces being connected together is extremely limited or in some cases not possible. Various particular applications impose unique, stringent requirements for which the fastener must be particularly adapted.
The application in which rivets or fasteners are used in the construction of aerodynamic designs, aircraft and the like, impose some of the most stringent requirements. In particular, the fasteners must secure the members but resist losing their gripping powe under the stresses and vibrations imposed upon them by the harsh environment in which they are used. It is desirable that such fasteners produce a bulbed-like configuration on the blind-side, or non-accessible side, of the workpieces being joined.
U.S. Pat. No. 3,253,495 discloses an assembly consisting of a pin or stem having a head on the blind-side thereof and pull-grooves on the accessible-side thereof, which fits into a sleeve the blind-side portion of which is deformable into a bulbed-like shaped head. To use, the blind bolt assembly is inserted into aligned boreholes of workpieces and a special tool having a plurality of chuck jaws grips the pull-grooves and pulls the pin outwardly away from the nonaccessible workpiece, thereby deforming the sleeve on the blind-side and forming a bulbed-like configuration. Unfortunately this fastener has several disadvantages. First the fastener requires a relatively expensive tool to effect its setting. Furthermore such a tool generally requires a relatively large amount of free-space on the accessible-side in order to grip the pin securely and complete the pulling operation required to set the fastener and form a tightly secured joint. In many situations in the aircraft industry, there is not available sufficient space on the accessible side to accommodate such special tools if such are required for the specific installation. Joints which are not tight must have the fastener drilled out and another joining operation attempted. Such corrective measures greatly increase the cost of fabrication.
Thus it is desirable to have a blind fastener which can be set without the need for specially designed tools and in fact, which can be set with readily available, relatively inexpensive, hand or power tools.
Other disadvantages of the blind bolt of U.S. Pat. No. 2,253,495 are that it has a relatively low preload retention, the importance of which will be subsequently explained, and it has a tendency to loosen its locking collar thereby reducing the gripping power of the bolt and increasing the deterioration of the joint through loss of the stem with subsequent loss of radial rigidity and continual exposure to vibration over longer periods of time.
Another blind rivet, disclosed in U.S. Pat. No. 4,012,984, attempts to overcome the problem of losing the locking collar by providing a locking groove in the stem or pin of the rivet. This groove which is of a smaller diameter than the main body of the stem, is designed to hold the locking collar in the assembled rivet after it is set. The features of the locking collar and its corresponding groove described in U.S. Pat. No. 4,012,984 are useful in this invention and accordingly the relevant teachings of U.S. Pat. No. 4,012,984 is hereby incorporated herein by reference. As in U.S. Pat. No. 3,253,495, U.S. Pat. No. 4,012,984 also has the serious disadvantage of requiring a special tool for gripping the stem portion of the rivet in order to set the rivet.
Both U.S. Pat. No. 3,253,495 and 4,012,984 employ break grooves which enable the surplus portion of the stem, after the rivet is set, to be broken away from the rivet upon further pulling by the special gripping tool. Unfortunately, a relatively rough surface is left on the accessible side of the rivet (as implied by FIG. 5 of U.S. Pat. No. 4,012,984) which is aerodynamically undesirable in streamlined surfaces employed in the aircraft and aerospace industries. Furthermore rivets which are broken totally by tension on the stem with pull tools such as those employed in both U.S. Pat. No. 3,253,495 and 4,012,984 produce an undesirable shock load on the fastener when the stem breaks, due to pin recoil which results in a relatively low preload retention. This problem could, of course, be eliminated in the blind rivet of these types by cutting off the stem rather than rupturing in a stem-pull operation as currently employed. However, it can be appreciated that the added task of cutting off the stem of the rivet is both expensive and time consuming, and in some cases, not possible because of insufficient working space on the accessible side of the rivet.
The blind fastener described in assignee's drawing PLT-1040 consists of a flush nut which is internally threaded over its entire longitudinal length, a deformable sleeve, a core bolt and a drive-nut. After the fastener is set the drive-nut is unscrewed from the core bolt and the stem is cut off and milled flush. Although this fastener has the advantage of eliminating stem or pin recoil it suffers from the disadvantage of requiring an expensive final milling operation to produce a smooth aerodynamically-acceptable surface. Fasteners such as the PLT-1040, are constructed from a high temperature A-286 alloy, and are used on honeycomb structures which can not withstand large amounts of compressive force or "clamp-up." After the fastener is set, the drive-nut is spun off (removed) from the stem member, the excess stem member clipped off, and the remaining stem member milled flush with the outer top surface of the flush-nut.
In order to prevent the removable portion of the stem member or "pin-tails" from inadvertently ending up in a piece of machinery, or laying about the aircraft structure so that it could possibly be sucked into the aircraft engine, thereby seriously damaging such machinery or engine, the aircraft industry laborously accounts for every pin-tail. Since the A-286 alloy and similar high strength, high temperature, alloys are nonmagnetic, a magnetic sweep of the aircraft structure and assembly area to recover the pin-tails is not possible because the drive-nut (which is magnetic) has been removed before the pin-tail is clipped. Thus it is desirable to have a fastener, which when the pin-tail is severed, has the drive-nut still attached to the pin-tail thereby allowing the pin-tail to be recovered by magnetic sweeping. There is another important advantage in having the drive-nut remain with its pin-tail, namely, since the pin-tail is relatively small (and the drive-nut relatively large) the pin-tail can become lodged in small openings from which it is very difficult to discover and effect its recovery.
In pull-type fasteners such as U.S. Pat. Nos. 3,253,495 and 4,012,984 the force exerted on the workpieces by the fastener just before the stem ruptures, referred to as "clamp-up", is significantly higher than the force exerted on the workpieces after the stem is ruptured at the break groove, referred to as "preload retention." This difference or loss of clamping force is attributed to stem recoil occuring at the time of stem rupture. The greater the stem recoil therefore the lower will be the force exerted on the workpieces by the fastener. Accordingly, it is desirable to sever the stem at the break groove in such a manner that stem recoil is eliminated or greatly reduced thereby resulting in higher preload retention.
Further disadvantages of other prior art fasteners are set forth in my copending application, Ser. No. 242,892, commonly assigned, which is hereby incorporated herein by reference.
Accordingly, there is a need for a method of installing a blind fastener with commonly available, relatively inexpensive, hand tools in which the surplus stem-portion can be severed without stem pulling and without expensive post-setting machining such as milling while at the same time producing an aerodynamically smooth surface at the severed stem and a joint having a relatively high preload retention. Such fasteners should induce screw failure at the faying surface of the drive-nut and nut body regardless of the grip condition of the fastener.